Simultaneous nitrification and denitrification (SND) in a single tank is highly desirable compared to the conventional systems, since separate tanks and recycling of mixed liquor nitrate from the aerobic nitrifying zone to the anoxic denitrifying zone is not required. The benefits of SND are further extended by exploiting the nitrite shunt pathway as has been demonstrated by the use of aeration duration control with ORP (see Guo et al. 2009, the disclosure of which is expressly incorporated by reference herein in its entirety) and ammonia pH profile (see Peng, et al. 2004, the disclosure of which is expressly incorporated by reference herein in its entirety). The reactor microenvironments (aerobic and anoxic zones developing within reactor due to combination of poor mixing and reactor design) and the floc microenvironments (aerobic an anoxic zones developing within the activated sludge flocs) have been postulated as possible mechanisms for SND (see Daigger et al. 2007, the disclosure of which is expressly incorporated by reference herein in its entirety). It is difficult to incorporate control strategies in the above-mentioned mechanisms to achieve stable SND performance. The occurrence of SND are reported in staged, closed loop reactors (such as oxidation ditch, orbal) (see Daigger and Littenton, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety) that typically employ long hydraulic residence time (HRT), solids retention time (SRT), and continuous low dissolved oxygen (DO).
The inhibition of nitrite oxidizing bacteria (NOB) is a precondition for the implementation of short-cut biological nitrogen removal (ScBNR) processes such as nitritation-denitritation (see Ciudad et al., 2005; Gee and Kim, 2004, Ju et al., 2007, Yoo et al., 1999, Yu et al., 2000, Zeng et al., 2008, the disclosures of which are expressly incorporated by reference herein in their entirety), nitrite-shunt and partial nitritation-anammox (see Fux et al., 2002, Hippen et al., 1997, van Dongen et al., 2001, Wett, 2006, Wett, 2007, Wett et al., 2010, the disclosures of which are expressly incorporated by reference herein in their entirety), and deammonification. Successful suppression of nitrite oxidation by controlling NOB saves 25% oxygen and 40% organic carbon compared to conventional nitrification-denitrification (see Turk and Mavinic, 1986; Abeling and Seyfried, 1992, the disclosures of which are expressly incorporated by reference herein in their entirety). In deammonification processes, the control of NOB results in added benefits in further reductions in aeration energy required, and reduced costs of electron donor and solids handling. FIG. 1, FIG. 2 and FIG. 3 show flowcharts for nitrogen removal through conventional nitrification/denitrification, nitritation/denitritation and deammonification (partial nitritation+anaerobic ammonia oxidation), respectively.
In view of high cost of biological nutrient removal (BNR) to meet increasingly stringent effluent standards, ScBNR through repression of NOB is a topic of interest. Efforts to understand NOB repression have been discussed in many publications, including those that are more specific to the use of high temperature (see Hellinga et al., 1998, the disclosure of which is expressly incorporated by reference herein in its entirety), high levels of free ammonia inhibition, or dissolved oxygen (DO) concentration (see Blackburne et al., 2008, the disclosure of which is expressly incorporated by reference herein in its entirety) and transient anoxia (see Kornaros and Dokianakis, 2010, the disclosure of which is expressly incorporated by reference herein in its entirety). Particularly, all of these conditions are used in part or as a whole, in various approaches, with success in controlling NOB in systems treating ‘high strength’ (high free ammonia) waste streams, such as anaerobic digester dewatering liquor (also usually at high temperature) and landfill leachate. Control of NOB repression in low strength waste streams such as domestic wastewater remains a challenge and is the subject of this disclosure. Controls that are currently used to repress NOB in ScBNR processes are described below.
Temperature and Ammonia:
Both temperature and free ammonia are features believed to provide an advantage to ammonia oxidizing bacteria (AOB) over NOB. Free ammonia (FA) inhibition of NOB has been well-documented in literature ever since it was considered by Anthonisen et al. (1976), the disclosure of which is expressly incorporated by reference herein in its entirety. However, knowledge of controlling FA inhibition to obtain stable nitritation is more limited since NOB adaptation has been reported (see Turk and Mavinic, 1989; and Wong-Chong and Loehr, 1978, the disclosures of which are expressly incorporated by reference herein in their entirety). Further, high temperature is known to favor growth of AOB over NOB (see Kim et al., 2008, the disclosure of which is expressly incorporated by reference herein in its entirety).
The increased activity of AOB compared to NOB at higher temperature, greater disassociation of total ammonia to free ammonia and resulting NOB inhibition at higher temperatures, combined with low DO operation (often conducted using intermittent aeration and with managed aerobic solids retention time (SRT)), results in enrichment of AOB and selective wash out of NOB. These approaches are variously described (see EP 0826639 A1, EP 0872451 B1, US 2010/0233777 A1, U.S. Pat. No. 7,846,334 B2, U.S. Pat. No. 6,485,646 B1, and WO 2012/052443 A1, the disclosures of which are expressly incorporated by reference herein in their entirety) to control NOB in ‘high strength’ wastewater. These methods either use suspended growth (see WO 2006/129132 A1, the disclosure of which is expressly incorporated by reference herein in its entirety), attached growth on the support media (see US 2011/0253625 A1 and EP 0931768 B1, the disclosures of which are expressly incorporated by reference herein in their entirety) or granular sludge (see Wett, 2007; and U.S. Pat. No. 7,846,334 B2, the disclosures of which are expressly incorporated by reference herein in their entirety) to accomplish ScBNR.
In spite of being effective, the role of elevated temperature to increase activity of AOB and for the control of NOB growth is not feasible in low strength mainstream processes operating under wide range of temperatures. Consequently, NOB control in low strength wastewater remains intractable and requires careful manipulation of factors other than temperature or free ammonia.
Dissolved Oxygen:
Dissolved oxygen (DO) can play a significant role in control of NOB in low strength wastewater. Sustained nitritation with the use of low DO concentration has been observed in a variety of reactor configurations (see Slickers et al., 2005, Wyffels et al., 2004, and Blackburne et al., 2008, the disclosures of which are expressly incorporated by reference herein in their entirety). Although, all of these reports lack account of underlying mechanisms, they resort to a hypothesis of higher oxygen affinity of AOB compared to the NOB (see Hanaki et al., 1990; Laanbroek and Gerards, 1993; and Bernet et al., 2001, the disclosures of which are expressly incorporated by reference herein in their entirety) as an explanation for the observed phenomenon (see Yoo et al., 1999, Peng et al., 2007, Lemaire et al., 2008, Gao et al., 2009, and Zeng et al., 2009, the disclosures of which are expressly incorporated by reference herein in their entirety). In a study Sin et al. (2008), the disclosure of which is expressly incorporated by reference herein in its entirety, has documented the prevalence of the belief that AOB oxygen affinity is greater than NOB oxygen affinity and that low DO operation favors AOB over NOB, however, there are studies that report to the opposite (see Daebel et al., 2007, and Manser et al., 2005, the disclosures of which are expressly incorporated by reference herein in their entirety).
Transient Anoxia:
The use of transient anoxia has been a common approach to achieve NOB suppression (see Li et al., 2012; Ling, 2009, Pollice et al., 2002, Zekker et al., 2012, U.S. Pat. No. 7,846,334 B2, EP 0872451 B1, and WO 2006/129132 A1, the disclosures of which are expressly incorporated by reference herein in their entirety). Transient anoxia allows for a measured approach to control the aerobic SRT as well as to introduce a lag-time for NOB to transition from the anoxic to aerobic environment. Kornaros and Dokianakis (2010), the disclosures of which are expressly incorporated by reference herein in their entirety, showed delay in NOB recovery and NOB lag adaptation in aerobic conditions following transient anoxia, thus confirming the observations of the usefulness of transient anoxia by many others (see Allenman and Irvine, 1980, Katsogiannis et al., 2003, Sedlak, 199, Silverstein and Schroeder, 1983, Yang and Yang, 2011, and Yoo et al., 1999, the disclosures of which are expressly incorporated by reference herein in their entirety). Although transient anoxia has been used successfully to control NOB in ‘high strength’ wastes (see Wett, 2007; and U.S. Pat. No. 7,846,334 B2, the disclosures of which are expressly incorporated by reference herein in their entirety) and the ability to use it in low strength wastes has been suggested (see Peng et al., 2004, the disclosure of which is expressly incorporated by reference herein in its entirety), the ability to control the features associated with transient anoxia remains an enigma. To summarize, strategies for controlling NOB repression in low strength wastewater, which is the basis for emerging ScBNR technologies, vary widely and a need still exists for more effective control strategies.